The present invention relates to a method and apparatus for analyzing the characteristics of a surface and more particularly for determining the adhering properties of a solid surface.
Understanding what happens on surfaces when materials interact one each other is fundamental in many industrial processes. Many assembly operations in microelectronics manufacturing need a precise assessment of the adhering properties of a surface, particularly in all those cases which involve the joining of two surfaces or the coating of a surface. For example metal surfaces might need to be cleaned and etched before being used in processes like soldering, electroplating, bonding or painting. A check of surface characteristics may also be required for silicon semiconductors before covering with protective resins. Another example of an operation needing an assessment of adhesion is the bonding of heat-sink to a device in order to predict the reliability of the joining.
Another example is represented by manufacturing processes requiring plastic parts to be painted or coated, not only for decorative purposes, but also for a number of functional reasons, e.g. for improving mechanical and chemical resistance (i.e. anti-scratch and anti-reflection coating for CR-39 optical lens), ESD and/or MFI shielding (polymer medullization).
Very often, plastic surfaces have a poor tendency to bond to other materials and offer low adhesion characteristics, making the painting process of the surface very difficult, because of the bad interaction of the plastic surface with the coating layer. The main reason for the low adhesion of plastic surfaces is the inherent inert chemical structure of polymers, which gives a low surface energy, and the presence of additives in their chemical composition, such as plasticizers, antioxidants and antistatics.
For the above reasons, a precise assessment of the adhering properties of a surface before its use is very important.
These properties are generally related to the chemical nature of the surface, but in the case of inorganic surface they are strictly dependent on the surface cleanliness. Many circumstances require that the cleanliness of a surface is tested and measured to verify that the contamination of the part is contained within acceptable limits. Furthermore, a check on the cleanliness of a surface may be required when a part is moved between two following steps and handling or transportation is required. For example a cleaning or a check of the degree of cleanliness may be required before electroplating of a part, or before a component is soldered onto a plated pad. An example is the mounting of a chip (device) on a substrate, usually done through soldering: this is called xe2x80x9cfirst level packagingxe2x80x9d. This stage of the process needs to be performed in a xe2x80x9ccleanxe2x80x9d environment to avoid contamination of the parts, before the module is encapsulated, usually with a resin, and the circuits are protected by external agents.
In the manufacturing of electronic components and products, the contamination of a surface can be caused by a number of different factors. Examples of causes of contamination are particulates, i.e. through the deposition of small particles; ionic contamination; deposition of chemical compound layers (e.g. oils or salts) occurring during the manufacturing steps; adsorption of organic material (e.g. hydrocarbons or moisture) caused by the exposition to the atmosphere.
If a degree of surface adhesion must be ensured, a reliable method of measurement of such degree has to be fixed.
One of the state of the art methods used to measure the adhering properties of a surface is the so called xe2x80x9ccontact anglexe2x80x9d method. The contact angle method is based on the measurement of the surface wettability. The contact angle is the angle between the substrate surface and the tangent of a liquid droplet deposited on the substrate at the point of contact of the liquid droplet with the substrate surface. This contact angle depends on the surface wettability. An ideal perfect wettability would cause the droplet to spread out over the substrate, giving a contact angle next to 0 degrees. A good wettability value would indicate a clean surface, while a bad wettability is symptom of organic contamination of the surface. By detecting any displacement on the wettability value from the expected one, an accurate estimate of the cleanliness degree can be derived.
It is known to measure this contact angle by projecting a profile image of the deposited droplet on a screen and estimating the contact angle by measurement on the projected image. One problem with this method is that the measurement of the angle is subject to significant errors and it is unreliable because of human intervention.
U.S. Pat. No. 5,268,733 describes a method for determining the contact angle by measuring, on a projected image of the droplet, the angle between a base line defined by the substrate surface and a reference line, which connects the contact point of the droplet with the substrate and an apex point of the droplet. This method provides a more reliable measurement of the contact angle, but still suffers of the difficulty of the measurement and of the human intervention in the determination of the angle.
Another drawback of the xe2x80x9ccontact anglexe2x80x9d method is that it is applicable only to flat and smooth surfaces, otherwise the irregularities of the surface would affect the tangent angle measurement.
Yet another drawback of the xe2x80x9ccontact anglexe2x80x9d method of the prior art is that the measurement starts from the assumption of an ideal droplet whose profile is an arc of circumference, while in the real world the profile is affected by the gravity force on the droplet liquid. FIG. 1a shows the effect of the gravity on a droplet and the corresponding angle of contact, compared to the ideal represented in FIG. 1b. To minimize the gravity influence on the droplet shape, the volume of the test droplet should be decreased to the order of xcexcl, but this would add serious difficulties to dispensing and measuring operations.
For the above reasons a more accurate and reliable method would be highly desirable. It is an object of the present invention to provide a technique which overcomes the above drawbacks.
According to the present invention, we provide a method for analysing the characteristics of a surface of an article, comprising the steps of maintaining the article so that the surface faces downwards, incrementally dispensing a volume of liquid against the surface, and determining a measure indicative of the gravity force necessary to overcome the adhesion forces between the liquid and the surface.
Also according to the present invention we provide an apparatus for estimating the adhesiveness of a surface of an article, the apparatus comprising means for maintaining the article so that the surface faces downwards, means for incrementally dispensing a volume of liquid against the surface, and means for determining a measure indicative of the gravity force necessary to overcome the adhesion forces between the liquid and the surface.